A riot has broken out at HMP Ranby in Nottinghamshire after up to 120 prisoners refused to return to their cells.
The Prison Officers’ Association (POA) said the “serious incident” began about 12.30pm and is continuing.
A spokesman for the Prison Service saidno staff or prisoners have been injured during the riot.
Negotiators were sent to the jailafter prisoners lit a fire and took control of a section of the prison after lunch.
Glyn Travis, assistant secretary of the POA, said: “There is a serious incident at HMP Ranby.
“At around 12.30pm, 120 prisoners refused to return to their cells. They have taken control of a unit.
“We have got national resources at the prison trying to establish a surrender plan. We know 60 prisoners have been actively involved.”
A report released on Thursday described HMP Ranby as unsafe, adding that there were high levels of violence at the prison.
The report followed an 11-day inspection by HM Inspectorate of Prisons.
It found that two prisoners suffered “self-inflicted” deaths last year, while another two died in a similar way last March.
Conditions at the prison were also described as dirty and nearly half the prison’s population said they felt unsafe.
The report found a growing number of self-harm incidents at the category C training prison, and the availability of legal highs was also found to have increased.
Chief Inspector of Prisons Nick Hardwick said the report showed a prison “in crisis”.